Electronically controlled fuel injectors for internal combustion engines, such as disclosed in U.S. Pat. No. 4,129,254 to Bader, Jr. et al, have enjoyed only limited acceptance despite their promise of significantly improved fuel efficiency and pollution abutment. A major factor tending to impede wider use has been the complexity and high cost of commercially available units. Only when major compromises in performance are tolerable have simplication and cost reduction been possible.
Recently, a simplified design for an electronically controlled fuel injector has been disclosed in U.S. Pat. No. 4,235,374 to Walter et al by which fuel injection may be both timed and metered electronically on a cycle-by-cycle basis using only a single electrically controlled valve. The '374 injector is characterized by a very simple mechanical structure including a cam operated primary pumping plunger and a secondary plunger hydraulically linked to the primary pumping plunger at selected times during the reciprocating motion of the primary pumping plunger. A single electronically controlled valve operates, upon closing, to form the hydraulic link and, upon opening, to break the hydraulic link by allowing liquid to flow freely into and out of the hydraulic link forming chamber. While quite simple in concept, extremely severe operating requirements are imposed on the electrically controlled valve since it must be capable of sealing back pressures in excess of 15,000 psi during injection periods and must be capable of accommodating sufficient flow volumes to allow the hydraulic link to be adjusted selectively on a cycle-by-cycle basis to produce the desired metering and timing functions. Moreover, a satisfactory valve must also be capable of moving between a fully closed position and a fully open position in less than 1 millisecond and must be relatively inexpensive to manufacture and failsafe in operation.
No previously known valve design is capable of meeting all of the above desired characteristics. For example, magnetically operated valves such as illustrated in U.S. Pat. No. 3,368,791 include a spool type valve plunger requiring expensive, high-tolerance machining operations and a relatively high inertia plunger and operator which would make the necessary speed of operation and flow handling capability difficult if not impossible to achieve. Lower cost ball-type valves (such as illustrated in U.S. Pat. Nos. 2,229,499; 2,792,195 and 3,464,668) are known but such valves also fail to provide high speed operation and satisfactory flow capacity while at the same time providing the necessary back flow sealing ability. For example, U.S. Pat. No. 2,792,195 to Mosbacher discloses a solenoid operated valve including a ball type valve element spring biased toward a closed position and moved to an open position by a spring biased valve operator which can only be rendered inoperative, thereby allowing the valve to close, when the operator is retracted against spring pressure by an electronically energized coil. While useful for the purposes intended, a Mosbacher type valve could not satisfy the operating criteria listed above because the valve operator relies on momentum build up rather than primarily spring force to move the ball element to its open position. Further increase in the size of the valve operator to permit a larger size spring would merely increase the inertia of the operator and work against achievement of the desired high speed operation. Reliance on the momentum imparted to the operator to open the ball valve also requires a significant gap between the valve operator and the valve to allow momentum build up upon de-energization of the coil. Such a large gap also works against high speed operation. Attempts to reduce this gap by increased spring force not only presents the inertia problem discussed above, but also introduces manufacturing tolerance which presents the possibility of improper valve closure. Still another problem is that a Mosbacher-type valve could probably not resist back pressures in excess of 1,500 psi and certainly could not resist back pressures in excess of 15,000 psi because of the use of O rings to seal the valve seat element.
In short, no valve design is known which would satisfy the operating characteristics imposed by a unit injector as discussed above while also meeting the requirement of low cost manufacture and reliability.